Antenna coupling system



AWM 2S, w36.V

L. R. KIRKWOOD ANTENNA COUPLING SYSTEM www Filed Jan. 30, 1954 Patented Apr. 28, 1936 ANTENNA COUPLING SYSTEM Loren R. Kirkwood, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 30, 1934, Serial No. 708,941

16 Claims.

The present invention relates to superheterodyne radio receivers and more particularly to an antenna coupling system for receivers of that character, and has for one of its objects to provide an improved signal input circuit for such receivers which is readily adapted for connection with differing antennas and for operation over differing wave bands or signal ranges.

Simplified multi-range superheterodyne receivers are being used extensively to provide a wider receiving range in low cost receivers. In receivers of this character the number of tubes is reduced to a minimum and in a number of circuits a single tube is arranged to have a plu rality of functions.

A reduction in the number of tubes required in a low cost superheterodyne receiver is most easily effected by reducing the number of radio frequency tuned amplifier stages preceding the rst detector to the point where the input signal is applied directly to the first detector. It is, therefore, a further object of the invention to provide a simplified input circuit of this character wherein the resulting loss in signal gain and selectivity may be compensated for to an appreciable degree.

It is also a further object of the present invention to provide an improved antenna coupling system which permits signals from an antenna or other energy collecting source to be applied directly to the first detector of a superheterodyne receiver, with minimum interference.

It is a further object of the present invention to provide an improved input circuit for a superheterodyne type receiver which tends to reduce interference from signals in adjacent wave bands, low frequency hum potentials and the like.

It is a still further object of the present invention to provide an antenna coupling system for the first detector of a superheterodyne receiver which permits the antenna circuit to track with the fundamental and the harmonic frequency of the oscillator, by a simple switching circuit, and to obtain constant gain at both ends of each tuning range provided in the receiver.

In accordance with the invention, an antenna or signal collecting circuit is coupled to a tuned signal input circuit for the receiver through one or more series condensers placed in the tuned circuit, and the oscillator is aligned to this circuit in such a manner as to provide the intermediate frequency at both the fundamental and second harmonic frequency thereof. A relatively high intermediate frequency is chosen. substantially below the signal range to be received, and the antenna circuit including a coupling choke coil (Cl. Z50-20) -frequency range covered by the second harmonic of the oscillator is found to have a smaller ratio 1o than the fundamental frequency range. Furthermore, for simplified tuning over a dual range the tuned input circuit is provided with a tapped tuning inductance and at the fundamental frequency will tune over a greater range than the Y15 second harmonic. Therefore, misalignment4 of the oscillator and main tuning condensers may result, as the latter are ganged for uni-control. For this reason means are provided for proper alignment of the two circuits, whereby the tuned input circuit in tuning through each range, will track with the oscillator circuit at both the fundamental and the harmonic frequency of the oscillator With ganged or uni-control tuning condensers.

The invention will, however, be better understood from the following description when con sidered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Figure 1 is a schematic circuit diagram of a radio receiver of the superheterodyne type provided with an improved input circuit embodying the invention, and Figure 2 is a View in elevation and substantially full size, of" a 35 preferred form of inductance unit providing the coupling windings of the input circuit of Figure 1.

Referring to Figure 1, 5 is an electric discharge device or tube of thecombination type providing a combined rst detector and oscillator for the receiving system shown, and is coupled through an intermediate frequency coupling transformer 6 with a similar combination device l providing a combined second detector and intermediate frequency amplier.

The combined detector-oscillator device 5 comprises a common cathode 8, an oscillator grid 9 and an oscillator plate electrode I connected with a tunable oscillator circuit Il which is provided with the usual variable tuning capacitor l2, and tuning and feedback windings I3 and I4 respectively. As the oscillator circuit is known and well understood, further description is believed to be unnecessary.

The device is further provided with a signal 55 input grid l5, a screen grid I6 therefor, and an output anode I1 which is connected to the primary winding |8 of the coupling transformer 6. The common cathode 8, grid |5, screen I6, and anode provide the first detector elements of the combination device 5.

The combined intermediate frequency amplier and second detector 'I is provided with a common cathode |9, an intermediate frequency signal input grid 20, a screen grid 2|, a suppressor grid 22 and an intermediate frequency output anode 23, forming a screen grid pentode for the amplification .of intermediate frequency signals. The secondary 24 of the intermediate frequency coupling transformer 6 is connected with the signal input grid 20 of the amplier,

Intermediate frequency signals from the output anode 23 are supplied through an untuned intermediate frequency coupling transformer 25, to a second control grid 25, in the device 1, being that of the second detector, and after detection by the anode bend method the audio frequency signals therefrom are taken through the output anode 2'I and an audio frequency output lead 28 to the remainder of the receiver circuit (not shown).

The cathodes 8 and |9 are connected through return leads 29 and 30 respectively to a common variable self-bias resistor 3| and a Xed bias resistor 32 to ground as indicated at 33, which forms the negative side of the anode supply circuit for the apparatus. The positive side of the anode supply circuit is indicated by the positive leadl 34 and is connected to the various tube electrodes, including the screen grids and anodes, as indicated. Bias connections are made to the bias resistors 3| and 32. Bias potential is applied to the rst detector circuit through means hereinafter described.

This system is, in general, shown and described, and the volume control and bias potential supply circuit is claimed in my co-pending application for Volume control circuit, Serial No. 704,709, led December 30, 1933, and assigned to the same assignee as this application and does not concern the present invention, further description of the system generally is believed to be unnecessary.

The intermediate frequency transformers 6 and 25 are designed to operate at a relatively high intermediate frequency, such as 460 kilocycles, for example which is substantially lower than the usual broadcast frequency range of from 550 to 1500 kilocycles. It will be noted that the transformer 25 is untuned while the transformer 6 is provided with both primary and secondary tuning capacitors 35 and 36 respectively for sharp intermediate frequency response at 460 kilocycles.

The relatively high intermediate frequency at 460 kilocycles is chosen for the reason that the first detector 5 is substantially directly connected with the energy collecting or antenna circuit of the receiver whereby a saving in the number of tubes employed in the receiver is effected. As has hereinbefore been pointed out, with this arrangement the selectivity is greatly lessened and image frequency response and interference from adjacent channel signals may result if ordinary coupling circuits are provided. The use of the high intermediate frequency is a material aid in preventing image frequency response and the selectivity of the circuit is further improved,

A4vvherea small number of tubes is employed in a superheterodyne receiver, by coupling the first detector with the antenna without the use of a transformer.

In the circuit shown the detector grid I5 is connected through an input lead 3`| with a tunable signal input circuit 38 comprising a tuning inductance 39 and a shunt variable tuning condenser 40, the latter being connected preferably to ground as indicated at 4|. The tuning inductance 39 is also connected to ground through two series connected condensers 42 and 43 for which the ground connection is indicated at 44. This arrangement places the main tuning condenser 40 in series with the condensers 42 and 43. It will be noted that the tuning condenser 40 is gang connected with the oscillator tuning condenser I2 for uni-control operation, as indicated by the dotted connecting line 45, being one unit of a multiple unit variable tuning condenser means.

Signals from the source or antenna, indicated at 46 are introduced into the tuned input circuit 38 through a coupling choke coil 41 directly in the antenna or signal collecting circuit between the antenna and ground 48. An antenna coupling condenser 49 is inserted between the choke '.f

coil 41 and the tuned input circuit and connection may be made with the latter selectively to an input terminal 50 between the condensers 42 and 43, or to an input terminal 5| connected with the low potential terminal 52 of the inductance 39, or between the inductance and the adjacent series condensers. The alternative connection is indicated by the dotted line 53 while the normal connection is indicated by the connection lead 54.

The inductance 39 is provided with a tap 55 intermediate between its end terminals 52 and 56, the latter being the high potential or grid end terminal. The tap 55 is connected with one contact 51 of a switch having a movable contact arm 58 and a second contact 59. The latter contact is connected to the terminal 50 and between the condensers 42 and 43 while the switch arm 58 is connected tol the terminal 5| and the low potential terminal end 5 2 of the tuning inductance 39.

It will be seen that operation of the switch serves to short circuit a portion of the inductance 39 when the arm 58 is moved to the contact 5l and alternatively to short circuit the condenser 42 when the arm 58 is moved to the contact 59 as shown.

The tap point 55 is so chosen on the tuning inductance 39 that the remaining portion thereof in circuit with the tuning condenser 40 will permit the input circuit to be turned over a predetermined desired high frequency range. In the present example the tuned input circuit may be assumed to tune over the normal broadcast range of 550 to 1500 kilocycles with the whole inductance 39 in circuit, and over the high frequency or short wave band from 1600 to 3500 kilocycles when the portion between the tap point 55 and the terminal 56 is in the tuned circuit.

Biasing potential for the rst detector control grid I5 is introduced in the tuned circuit above ground potential and at the low potential end or terminal 52 of the tuning inductance 39 through a supply lead 60 and a suitable lter impedance 6| from a source which in the present example is the variable self biasing resistor section 3| common to the cathode return leads 29 and 30 of the devices 5 and A filter condenser 62 on one side of the filter impedance 6| adjacent to the supply and the condensers 42 and 43 on the other side adjacent tothe grid circuit, complete the grid lter means for the bias supply connection to the grid l5.

The tuned signal input circuit 38 is therefore tunable over two differing frequency ranges and the change from one frequency range to the other is effected by a simple switching operation involving short circuiting a portion of the tuning inductance 39 for the higher frequency range. At the same time, the condenser 42 is placed in series with the tuning condenser 40 in the tuned circuit. The value of the condenser 42 is such that the oscillator condenser I2 and the main tuning condenser 40 may track as in the lower or broadcast tuning ranges throughout the high frequency tuning range, to produce the same intermediate frequency. The condenser 42 serves to decrease the range of the high frequency circuit which would otherwise be too wide, and is introduced in series with the tuned circuit 38 for that purpose.

When the antenna input connection is made to the terminal 5t, signals are applied to the tuned input circuit from the antenna choke coil 41 through the coupling condensers 49 and 43, the

latter being in the tuned input circuit. For the high frequency tuning range an appreciable increase in gain may be obtained by connecting the antenna input circuit to the terminal 5I as shown by the dotted line connection 53, so that both condensers 42 and 43 as well as the coupling condenser 49 are arranged in the antenna circuit and both condensers 42 and 43 are in the signal input circuit for the rst detector.

It will be noted that when the alternative connection to the input terminal 5I is employed, switching from the high frequency band with the switch arm at 51, to the broadcast band with the switch arm at 59 causes the alternative antenna connection in effect to be returned to the terminal 50 automatically. Therefore, the connection 53 may be maintained for short wave and for broadcast reception and the entire switching may automatically be completed by closure of the switching arm 58 with the contact 459 to provide the preferred broadcast connection for the antenna between the condensers 42 and 43.

The antenna coupling choke coil 41 is so designed that the combination of condensers 49 and 43 in series across it tunes it to a relatively low frequency below the lowest frequency band to be received. In the present example, with the broadcast frequency as the lowest frequency range to be received, the choke coil 41 is tuned to substantially 35 KC, the lowest broadcast band frequency being assumed to be 550 KC.

It has been found that in receiving short wave signals over a short wave or high frequency range of from 1600 to 3500 KC for example, with the usual broadcast frequency tuning elements comprising the inductance 39 and tuning condenser 40, the condenser 42 may have a value of substantially '700 micrornicrofarads and the condenser 43 may have a value of substantially 1500 micromicrofarads. The coupling condenser 49 may have a capacity of substantially .01 microfarads.

From the foregoing description, it will be seen that in connection with a rst detector of a superheterodyne receiver for direct signal input from a source, such as an antenna, there is provided an intermediate frequency output circuit tuned to an intermediate frequency below the lowest signal range to be received and an input circuit directly connected with an antenna or other sig nal collecting circuit comprising a tuning inductance and a shunt variable tuning capacity both connected to ground and with two series connected condensers between the inductance and the ground connection. Furthermore, there is provided a simple two point switching means for alternatively short circuiting one portion of the tuning inductance preferably at-the lower potential or ground end, and the series connected condenser adjacent thereto in the ground connection lead, while the antenna or signal input circuit, comprising a choke coil tuned by the second series condenser to relatively lowr frequency below the lowest frequency range to be received is connected to the said remaining series condenser or across both of the series connected condensers in the ground connection for the main tuning inductance.

Furthermore, bias potential supply connection is made to the tuning inductance at the low potential end thereof and between it andthe adjacent series condenser whereby the bias potential is introduced into the tuned circuit above ground potential.

With the above arrangement, the series connected condensers in the ground connection for the main tuning inductance, provide a combined antenna tuning capacity for reducing the antenna tuning to a relatively low frequency, a coupling capacity in the tuned input circuit and a filter capacity for the bias supply connection. Furthermore, the switching operation provides simultaneously for changing the wave length or tuning range of the inputcircuit and for effecting a change in the tuned input circuit whereby it may track with the second harmonic of the oscillator in a tuning system wherein the tuning capacities are ganged for uni-control operation.

It will be noted that the coupling choke 41 and the main tuning inductance 39 are not electro-magnetically coupled. However, they may bc arranged on a common winding form for cornpactness and for simplifying and reducing the cost of construction of the apparatus. A present preferred arrangement of the tuning inductance 39 and the antenna choke coil 41 is shown in Fig. 2. The inductance and choke coil bear the same reference numerals as in Fig. 1 and are shown mounted on a cylindrical form 63 in spaced relation to each other. The terminals 56, 55 and 52 correspond to the terminals indicated in Fig. 1, the terminal 55 being connected with the tap connection on the inductance 38, as indicated.

The spacing between the choke coil 41 and the winding or inductance 39 is preferably such that the electro-magnetic coupling is reduced to a relatively low value although it has been found that spacing substantially as shown may be employed with satisfactory results.

t will also be noted that on either side of the tap connection with the terminal 55, a shiftable turn 54 is provided in the winding for changing the inductance of the winding on either side of the tap to adjust the tuning for the high frequency and broadcast tuning ranges to an initial value. 'I'his method of changing the inductance of a winding is shown, described and claimed in DeTar, United States Patent #1,860,176.

It will be appreciatedV that any other suitable arrangement of the inductance 39 and the choke coil 41 may be provided, but the arrangement specifically shown inFigure 2 is at present preferred for compactness and low cost of production since it has been found that both the choke coil and the inductance may be mounted on the same form substantially adjacent to each other without interfering with the desired operation of the coupling circuit. It has been found that the connections to the choke coil may be reversed without affecting the operation, and accordingly no dependence for coupling is made upon the electro-magnetic relation of the two windings.

Referring again to Figure 1, the tuned circuit 38 will tune wiih a given frequency ratio with respect to the oscillator frequency depending upon the ratio of the condenser 43 to the variable condenser 49. This ratio is adjusted so that substantially equal amplification may be obtained over the normal or broadcast frequency range. Since the second harmonic of the oscillator frequency will change in the same ratio as a fundamental frequency and since the intermediate frequency is a certain proportion of the signal frequency range, the short wave band covered by the second harmonic of the oscillator frequency will be a smaller ratio than the fundamental range. Accordingly, the series condenser 42 is provided to properly align the two circuits so that the tuned circuit 38 will properly track with the oscillator circuit I l throughout the high frequency or short wave range, and in response to the second harmonic frequency, will produce the same intermediate frequency as for the broadcast range. By choosing a suitable series capacity for the condenser 42 and inductance in the winding 39, the circuit may be made to track throughout both frequency ranges without the addition of other corrective capacitors.

I claim as my invention:

1. An antenna coupling circuit for a superhetercdyne receiver having an oscillator and a tuning condenser therefor, comprising in combination, a main tuning inductance, a main variable tuning condenser connected therewith, said oscillator tuning condenser' being connected with said main variable tuning condenser for uni-control op eration therewith, a pair of condensers in series between said main tuning condenser and said tuning inductance, an an'enna input coupling choke coii connected in parallel with one of said sies connected condensers, and means for selectivel;7 short circuiting a portion of said main tuning inductance and the other of said series condensers.

2. An antenna coupling circuit for a superheterodyne receiver having an oscillator and a tuning condenser therefor, comprising in combination, a main tuning inductance, a main variable tuning condenser connected therewith, said oscillator tuning condenser being connected with said main variable tuning condenser for uni-control operation therewith, a pair of condensers in series between said main tuning condenser and said tuning inductance, an antenna input coupling choke coil, means for connecting said coil to a point between said series connected condensers, and means for changing the inductance value of said main tuning inductance and short circuiting one of said condensers.

3. An antenna coupling circuit for a superhei'erodyne receiver having an oscillator and a tuning condenser therefor, comprising in combination, a main tuning inductance, a main variable tuning condenser connected therewith, said oscillator tuning condenser being connected with Said main variable tuning condenser for uni-control operation therewith, a'pair of condensers conlterminal nected in series between said main tuning condenser and said tuning inductance, an antenna input coupling choke coil connected in parallel relation with one of said series connected condensers, and selective switching means providing connections for adjusting said main tuning inductance and alternatively short circuiting the other of said series connected condensers.

4. An antenna coupling circuit for a superheterodyne receiver having an oscillator and a tuning condenser therefor, comprising in combination, a main tuning inductance, a main variable tuning condenser connected therewith for tuning through one frequency range, said oscillator tuning condenser being connected with said main variable tuning condenser for uni-control operation therewith, a pair of condensers connected in series between said main tuning condenser and said tuning inductance, said last named pair of condensers providing capacity value in circuit to cause said variable condenser to track with said oscillator tuning condenser through another frequency range, an antenna input coupling choke coil connected in parallel relation with one of said series connected condensers, said choke coil being thereby tuned to a frequency below either of said differing tuning ranges and said antenna circuit being thereby coupled in circuit with said main tuning inductance and variable tuning condenser,

and selective switching means for adjusting said main tuning inductance and alternatively short circuiting the other of said series connected condensers.

5. An. antenna coupling circuit for a superheterodyne receiver having an oscillator and a tuning condenser therefor, comprising in combination, a main tuning inductance, a main variable tuning condenser connected therewith, said oscillator tuning condenser being connected with said main condenser for unicontrol operation therewith, a pair of condensers connected in series between said main tuning condenser and said tuning inductance, an antenna input coupling choke coil connected in parallel relation with one of said series connected condensers and selective switching means for short circuiting a portion of said main tuning inductance and alternatively short circuiting the other of said series connected condensers.

6. An antenna coupling circuit for a superheterodyne receiver having an oscillator and a tuning condenser therefor, comprising in combination, a main tuning inductance, a main variable tuning condenser connected therewith, said oscillator tuning condenser being connected with said main variable condenser for unicontrol operation therewith, a pair of condensers connected in series between said main tuning condenser and said tuning inductance, one of said condensers having a value to cause said variable tuning condenser to track with the oscillator tuning condenser at a higher frequency tuning range in response to a harmonic frequency of the oscillator, a coupling choke coil connected in parallel relation with one of said series connected condensers and selective switching means for short circuiting a portion of said main tuning inductance and alternatively short circuiting the other of said series connected condensers.

'7. In a superheterodyne receiver, the combination with an electric discharge device having a control grid, an anode and a cathode, of means for applying radio signals to said control grid, comprising a main tuning inductance having a connected with said control grid,

a tuning condenser having an electrode connected with said terminal, a connection with the cathode and a connection with a second electrode of said variable tuning condenser, a circuit tracking condenser and a coupling condenser connected in series between a second terminal of the main tuning inductance and said lead, the tracking condenser being adjacent to said second terminal, and means for selectively short circuiting a portion of the main tuning inductance adjacent to said last named terminal and alternatively short circuiting said last named condenser.

8. In a superheterodyne receiver, the combination with an electric discharge device having a control grid, an anode and a cathode, of means for applying radio signals to said control grid, comprising a main tuning inductance having a terminal connected with said control grid, a tuning condenser having an electrode connected with said terminal, a lead providing a connection with the cathode and a connection with a second electrode of said variable tuning condenser, a circuit tracking condenser and a coupling condenser connected in series between a second terminal of the main tuning inductance and said lead, the tracking condenser being adjacent to said second terminal, means including a supply lead and a filter impedance device therein connected with said second terminal to supply bias potential to said grid, and means for selectively short circuiting a portion of the main tuning inductance adjacent to said last named terminal and alternatively short` circuiting said last named condenser.

9. In a signal input circuit for a radio receiver, a tuning inductance, a tuning condenser connected in shunt relation thereto, a coupling condenser connected in series between said inductance and variable tuning condenser, an input choke coil connected substantially in parallel with said series condenser being tuned thereby to a frequency outside the tuning range of said tuning condenser and tuning inductance, a second series condenser, and selective switching means providing a connection for short circuiting a portion of said tuning inductance and inserting said last named condenser in series with said main tuning inductance and variable tuning condenser.

l0. In a signal input circuit for a radio receiver, a tuning inductance, a tuning condenser connected in shunt relation thereto, a coupling condenser connected in series between said inductance and variable tuning condenser, an input choke coil connected substantially in parallel with said series condenser and being tuned thereby to a frequency outside the tuning range of said variable tuning condenser and tuning inductance, said coupling choke coil comprising a compact multi-layer winding, and common means for supporting said inductance and choke coil in spaced relation to each other as an inductance unit.

ll. An antenna coupling circuit for a superheterodyne receiver having an oscillator and a tuning condenser therefor, comprising in comblnation, a main tuning inductance having a tap between its terminal ends, a variable tuning condenser connected therewith, said oscillator tuning condenser being connected with said main Variable tuning condenser for uni-control operation therewith, a pair of condensers connected in series between said tuning condenser and said tuning inductance at the low potential end of said last named inductance, an antenna coupling a lead providing choke coil, a series coupling condenser, means for selectively connecting said antenna circuit through said coupling condenser to a connection point between said series connected condensers and a second point between one of said condensers and the main tuning inductance, and selective switching means for connecting the low potential terminal of said tuning inductance to said first named point and alternatively to said tap on the main tuning inductance.

12. In a superheterodyne receiver, the combination with a first detector and an oscillator, of uni-control variable condenser means for tuning said oscillator and detector, a tunable detector input circuit including a unit of said variable condenser means and a tapped tuning inductance responsive to signals over a predetermined frequency range and said oscillator being tunable therewith over a frequency range to produce a beat or intermediate frequency below said last named frequency range, means providing a series capacity for said tunable circuit, switching means providing connections for simultaneously short circuiting a portion of said inductance and inserting said series capacity in said tunable circuit, the resulting inductance being lowered and said series capacity being of a value to limit the tuning range of said circuit, whereby said circuit is responsive to signals within a higher frequency range adapted to beat with the second harmonic frequency orf said oscillator to produce the same intermediate frequency, means providing a second capacity in said tuned circuit in series with said first named capacity, a choke coil connected substantially in parallel with said last named capacity, said choke coil and last named capacity providing a tuned circuit having a frequency response substantially lower than the lowest frequency in said rst named tuning range, and means for applying signals across said choke coil.

13. The combination with a first detector for a superheterodyne receiver and an oscillator, of an antenna coupling system for said detector comprising a tuning inductance, a variable condenser connected for uni-contro-l tuning with seid oscillator, two condensers connected in series between said variable condenser and said inductance, and means for applying signal potentials across at least one of said condensers, comprising a coupling choke coil connected substantially in parallel with said last named condenser and mounted as a unit with and in spaced relation to said tuning inductance, said choke coil being separately tunable by said last named condenser to a frequency below the lowest frequency in the tuning range of said tunable input circuit.

14. The combination with a rst detector for a superheterodyne receiver and oscillator tunable'to provide fundamental and harmonic frequency oscillations in predetermined frequency ranges, o-f an antenna coup-ling system for said detector, including a tunable input circuit connected with said detector and comprising a tuning inductance, a variable condenser connected for uni-control tuning with said oscillator, and two condensers connected in series between said variable condenser and said inductance, switching means for selectively short circuiting one of said condensers and a portion of said tuning inductance, and means for applying signal currents to said tunable input circuit across at least one of said two condensers, comprising a coupling choke coil connected substantially in parallel with at least one of said two condensers and tunable thereby to a. frequency below the lowest frequency in the tuning range of said tunable input circuit, said tuning inductance and said choke coil being mounted on a common form in spaced relation to each other, said tuning inductance having a tap, and means for adjusting the inductance on either side of said tap.

15. In a. superheterodyne receiver, the combination with a first detector, an oscillator, and a multiple u nit variable condenser means for simultaneous tuning control of said detector and oscillator, of a tunable antenna coupling circuit for said rst detector, said circuit including a unit of said variable condenser means and shunt tuning inductance tunable with said condenser unit through a predetermined frequency range, a series tracking condenser for said circuit, switching means for connecting said tracking condenser in said circuit and for short circuiting a portion of said tuning inductance whereby said tuned circuit is tunable over a differing higher frequency range by said variable condensei` means to produce the same intermediate frequency, a coupling condenser connected in series with said tunable circuit between said tracking condenser and said variable condenser unit, and an input coupling choke coil connected substantially in parallel with said coupling condenser and being tunable thereby to a frequency below the lowest signal frequency to be received by said tunable circuit.

16. In a superheterodyne receiver, the combination with a first detector and an oscillator, of a tunable antenna, coupling circuit for said rst detector, said circuit including a variable tuning condenser and shunt tuning nductance tunable with said condenser through a predetermined frequency range, said tuning condenser being one of several interconnected for simultaneous tuning operation of the several circuits of the receiver, a series tracking condenser for said circuit, swtching means for connecting said condenser in said circuit and for short circuiting a portion of said tuning inductance whereby said tuned circuit is tunable over a diiering higher frequency range by said variable tuning condenser to produce the same intermediate frequency, a coupling condenser connected in series with said tunable circuit between said tracking condenser and said variable tuning condenser, an input coupling choke coil connected substantially in parallel with said coupling condenser and tunable thereby to a frequency below the lowest signal frequency to be received by said tunable circuit,'and means for supplying the biasing potential to said detector between said tracking condenser and the tuning inductance, said last named means including a supply lead and a lter impedance therein.

LOREN R. KIRKWOOD. 

